Abstract: Based on the potential energy method, a semi-discrete method is presented in this paper. The method can save much computing time than the standard finite element method and even the finite strip method. By abandoning one of the Vlasov's assumption in analysis of thin-walled members, which assumption is that the shear strains along the central line of the cross section are zero or constant, the present method can show the shear lag effect well. In this paper, the longitudinal displacement along the cross section is interpolated by a new proposed sectional spline function. By using the variational principle, a group of ordinary differential equations and natural boundary conditions can be derived. By solving these equations an analytical solution for the longitudinal displacement along the member axis can be obtained. The present method is suitable for any shape of cross section thin-walled members. Some typical numerical examples in this paper demonstrate the versatility, efficiency, accuracy and convergency of the proposed method.